1. Field of the Invention
The present invention relates to a magnetic resonance imaging technique for imaging the inside of a subject on the basis of a magnetic resonance phenomenon occurring in the subject. More particularly, this invention is concerned with a magnetic resonance imaging (MRI) system and magnetic resonance (MR) imaging method based on the time of flight effect of nuclear spins residing in blood flow passing through two-dimensional planes to be imaged of the subject.
2. Description of the Related Art
Magnetic resonance imaging is a technique for magnetically exciting nuclear spins in a subject positioned in a static magnetic field by applying a radio-frequency signal with the Larmor frequency, and reconstructing an image using an MR signal induced with the excitation or providing a spectrum of an MR signal.
In the MRI field, techniques for imaging blood flow of a subject have already been in practical use in medical examination. One blood flow imaging technique is based on the two-dimensional time of flight(2D-TOF) effect, which utilizes simple changes in positions of nuclear spins existing within blood vessels. Using this technique, two-dimensional images associated with blood flow are produced and maximum intensity projection (MIP) images and/or rotational images of blood vessels are obtained from three-dimensional images formed by virtually piling up a plurality of two-dimensional images.
The two-dimensional time of flight (2D-TOF) method is particularly effective when flows of blood are perpendicular or thereabout to a sliced plane to be imaged, and adequate for the observation of blood flow over the wider range in the body axis direction of a subject. Pulse sequences used in scanning to produce the 2D-TOF use a rephase pulse for imaging two-dimensional regions at specified slice positions.
In human bodies taken as the subject, the speed of blood flow in peripheral blood vessels is slower. In such case, the depiction of blood flow based on the time of flight effect can be performed by adjusting the slice thickness values and flip angles in imaging.
According to the 2D-TOF method, for example, when the blood flow speed, as in blood flow through the inferior limb, is slower, relatively higher flip angles (nearer to 90 degrees) are employed to raise contrast levels. However, the higher the flip angle becomes, the larger are the intensities of an MR signal induced from surrounding fat of vessels, thus providing a problem in that the MR signal from the fat should be removed for carrying out maximum intensity projection processes employed as a one post-process. If the MR signals from fat are higher in intensities, then they are modulated into higher brightness values, providing a problem in that the MR signal from fat is excessively enhanced and the like, and thus deteriorating the depiction ability of blood vessels. In the 2D-TOF method, if a region to be imaged is narrow, the MR signal from fat can suitably be suppressed; on one hand, if the region is wide, it is almost impossible to suitably suppress the MR signal from fat in a uniform manner over the wide region.
There is a countermeasure to suppress the MR signal from fat. Prior to applying a pulse sequence for acquiring MR signals for image reconstruction (frequently, referred to as a main scan), a pre-pulse is applied for suppressing the MR signal to be induced from fat. However, mere application of the pre-pulse will lead to poor resolution and/or visibility of blood flow images.